As a long time user of Fritz my first experience with Aquarium was unfavorable because compared to Fritz, the GUI was way too complicated for my taste. The Aquarium interface has so many bells and whistles it takes a while to learn how to use it. I ended up setting up a practice database with a few games just so I could practice
I used the download version from ChessOK and had some problems. With my first experience the serial number would not work. Their tech support got back to me quickly and advised that the problem was not with the program, but rather with the way Windows copied and pasted. All I had to do was type the serial number in and the download then went smoothly.
The second experience also ran into download problems. The download hung up and after several tries I got a message that the limit on downloads had been reached. An e-mail to tech support (specifically, fellow named Alexander Zhuravlev) got answered fairly quickly considering my inquiry was sent out on Thursday evening; I heard back on Saturday. I was given another link with unlimited downloads and things went quickly and smoothly.
I have heard of some bugs in the program and a few complaints, apparently mostly from new users, about the complexity of the program and some bugs, but I have not run into any real bugs. This is probably because I have used the download version with the bug fixes included. Buying a CD may have any bugs in the program included.
There are a several things that I find more complicated than Fritz, but I am slowly learning and Aquarium is growing on me. Like everybody, I am finding some things I like and some I don’t; no program is perfect, I guess.
One thing I do not like are their chess publishing tools. Copying a game into HTML for a blog runs to 150 pages! I just use the Knight Vision site. All in all, I am finding this program does indeed have a lot of features that are absent in Fritz and Aquarium is slowly becoming my chess program of choice as I find myself using Fritz less and less.